The present invention relates to a process for the production of alginate fibres, particularly dried alginate fibre material, and to products made therefrom; it is especially, although not exclusively, useful for the commercial manufacture and preparation of materials and products, composed of alginate fibres, for use as swabs or dressings and the like suitable for medical, surgical and other purposes.
The extrusion of alignate solutions into coagulating baths or spin baths containing for example aqueous solutions providing calcium ions to form or spin yarns of insoluble alginate salt filaments is a well known process.
It is also well known to prepare and use materials or fabrics composed of alginate fibres, especially calcium and sodium/calcium mixed salt alginate fibres, for surgical and medical purposes, for instance in the form of dressings and the like in connection with which the hemastatic properties of the alginate and, in some cases, solubility in body fluids can be particularly valuable. Such uses are for example described in British Pat. Nos. 653341 and 1394741.
In the past, alginate fabrics for providing surgical and medical dressings and the like have generally been produced from continuous calcium alginate filament yarn by knitting. When the alginate material is required in the more soluble sodium/calcium mixed salt form, which is commonly preferred and is more useful in many cases, the knitted fabric is then treated or "converted" to replace part of the calcium content by the more solubilizing sodium cation.
This conversion of the calcium alginate to the more soluble sodium/calcium alginate mixed salt form is the subject of a number of Patent publications and an excellent summary is given in British Pat. No. 1394741 which sets out the basis for a process which is technically superior to previous methods. In practice, however, the treatment or conversion process performed on the knitted fabric has usually involved handling problems and has been carried out batchwise which is rather inefficient and uneconomic resulting in a costly and wasteful production process.
As will be seen later, the process of the present invention can enable calcium alginate fibres to be wet spun and processed to form a dried alginate fibre material in a continuous series of operations which can include, when required, a step of converting the calcium alginate fibre material into the sodium/calcium mixed salt form. The dried alginate fibre material so formed may then be made up into a tow suitable for swab production or alginate wool or into the form of a non-woven wadding suitable for use as a medical or surgical dressing. The entire process can be performed without handling the material, using simple machinery which is easy to maintain to the standards of cleanliness required of this type of product.
A factor to be considered, however, in the production and processing of alginate fibres is the tendency of such fibres, when wet, to adhere and fuse together on drying. This tendency is of major importance in the drying of wet newly-spun "never-dried" alginate fibres since these are extremely soft and have a very high water content so that, during drying, considerable dewatering takes place and adjacent individual fibres or filaments are caused to adhere to each other by capillary action and become bonded together. This bonding effect can be dramatically increased by any mechanical forces externally applied at the dewatering drying stage.
It may be noted that this tendency for inter-fibre bonding to occur is not so important in drying "re-wetted" alginate fibres, that is, alginate fibres or yarns which have been dried after being initially spun and which have then subsequently been re-wetted. In the latter case, provided a significant degree of fibre bonding has been avoided in first producing and drying the fibres it is found that on subsequent re-wetting considerably less water is taken up by the fibres and during re-drying the tendency for the fibres to adhere and bond together is very much reduced. On the other hand, in any continuous production process which starts with the initial step of wet spinning calcium alginate fibre filaments one is necessarily dealing with "never-dried" alginate fibres which remain wet up to the drying stage at which the tendency to adhere and bond together arises as discussed above.
In the commercial production of continuous dried alginate filament yarns the problem of excessive and undesirable bonding between individual fibre filaments may be overcome by coating the filaments before drying with a substance such as an emulsified oil which prevents the filaments from sticking together. Or, as an alternative remedy, fibre adhesion may be inhibited by exchanging and removing the water with an organic water soluble solvent such as acetone or a lower alcohol at a sufficiently high concentration.
It has been already-dried alginate filament yarns as referred to above which have previously been commonly used for making knitted alginate fabrics in the conventional processes for providing surgical and medical alginate dressings. It has, however, also been proposed in publication document WO 80/02300 of International Patent Application No. PCT/GB80/00066 (Courtaulds Limited) to provide a non-woven alginate fabric for use as an alginate dressing and a process for the production of such non-woven fabric is disclosed therein.
According to the process of WO 80/02300, a tow of stretched and washed spun calcium alginate fibres or filaments is passed in a flow of water through a spreading device, such as a device with a "fish tail" outlet, and the spread band or sheet produced is fed forwards and deposited on a liquid permeable conveyor, such as Fourdrinier wire mesh conveyor, moving at a slower speed so that the fibres are overlaid in a substantially uniform layer or sheet forming a web which is then dried to provide a unitary non-woven alginate fabric. As a result of the overlaying, the fibres become crimped or looped and cross over each other in the web so that a parallel orientation thereof is destroyed; it will be appreciated that this reduces the contact area of the fibres and prevents the formation, in the subsequent dewatering drying operation, of a sheet in which parallel fibres are bonded and fused together along their length. Such a sheet of parallel bonded alginate fibres would be completely unsuitable for medical or surgical alginate dressings.
The advantages resulting from overlaying the fibres as described above suggests that no other special measures need to be taken to counteract bonding together of the fibres. Indeed, on the contrary, the disclosure in WO 80/02300 indicates that some fibre bonding is desirable and particularly emphasizes that although lengthwise bonding is avoided the overlaid fibres in the web produced should still be bonded at their points of contact where they cross over each other; the drying operation in the process described accordingly is carried out in a way which applies mechanical forces and promotes such bonding at intersections, first by applying suction directly to the wet overlaid fibres on the conveyor and then by passing the partially dried web around heated cylinders. Also, a way of further increasing the degree of such inter-bonding by a treatment of the filaments on the conveyor with an aqueous solution of a sodium salt, preferably a sodium salt of an acid which forms on insoluble calcium salt, before completing the drying operation is described. A non-woven alginate fabric is thus produced by the process of WO 80/02300 consisting essentially of a dried web of intersecting alginate fibres or filaments bonded together at their cross-over points to provide a strong unitary structure.
The production of non-woven unitary webs or fabrics directly from tows of continuous synthetic textile filaments is also already known from U.S. Pat. No. 3,802,980 (Harmon) which describes a manufacturing process similar in some ways to that described in WO 80/02300, especially in respect of the use of a fish-tail type spreader to overfeed onto a more slowly moving liquid permeable conveyor a thin sheet or band of filaments or fibres so that the latter become crimped and overlaid in crossing-over or intersecting overlapping relationship in a continuous filament sheet which is then dried to produce the non-woven unitary web required. The fibres or filaments of the web produced are not described as being bonded and it is implied that frictional engagement between overlapping crimped or looped fibre portions of adjacent individual filaments suffices to maintain a unitary structure. This may be so, although it is notable that in the specific examples described the filament sheet on the conveyor is sprayed before final drying with a polyvinyl alcohol solution which is known to act as an adhesive and promote bonding to some extent with many fibres.
In any event, the process of U.S. Pat. No. 3,802,980 is not specifically described as being applied to alginate fibres or filaments but appears to be directed more to synthetic textile filaments such as viscose rayon, nylon, polyesters, acrylics and polyolefins etc., and it can be inferred that the initial tows to which it is expected that the process would be applied would be composed of already dried continuous filaments of these materials. If, on the other hand, the process were to be applied to "never dried" alginate fibres and carried out exactly as described using in particular a suction box to remove water from the freshly laid sheet or web on the conveyor, a high degree of bonding between the fibres or filaments at their cross-over points would inevitably be obtained as in the process of WO 80/02300. This, however, would be quite in accordance with the teaching of the latter and may therefore well be considered to represent a useful advantage.